The present invention relates to an apparatus and method for sensing the tilt of an underground storage tank (UST). In more detail, the present invention relates to a method and apparatus capable of sensing, from above ground, any deviation from level of an underground storage tank.
Although reference will be made throughout this specification to the testing of gasoline storage tanks because such tanks are the most common application of the method and apparatus of the present invention and the Patent Statute requires the disclosure of the preferred embodiment of the invention, those skilled in the art will recognize from this disclosure that the present invention is capable of application to any underground storage tank regardless of the type of liquid stored therein and/or the particular business in connection with which the UST is operated.
By federal regulation (53 C.F.R. .sctn..sctn.280 et seq.), the amount of gasoline pumped from an underground gasoline storage tank must continually be reconciled with the volume of gasoline in the tank and the volume of gasoline pumped into the tank. To do so, the filling station operator drops a dipstick into the tank through the tank fill pipe to measure the depth of the gasoline in the tank. Of course, for such a measurement to be an accurate indicator of the volume of gasoline contained in the tank, the gasoline in the tank must be of uniform depth throughout the tank. However, variations in the shape of the tank caused by the installation of the tank, changing temperature, water table depth, and other ambient conditions, and the volume of the liquid in the tank decrease the likelihood of uniform depth of the gasoline stored therein. Tank manufacturers supply a chart, referred to as a strapping chart, by which the volume of the liquid in the tank can be more accurately calculated by measurement of the depth of the liquid, but even the most carefully installed tank and the most accurate strapping charts will not provide accurate volume calculations if the tank is not level. Further complicating matters is that, in an ideal installation, the tank is installed with a tilt toward the end of the tank into which the fill pipe opens. This tilt amounts to approximately a 4" difference in the height of one end of the tank, compared to the other, along the length of a standard right angle cylinder ten thousand gallon tank. Of course given the volume of liquid contained in such a tank, such a tilt can throw off the volume calculation from a measurement of the depth of the liquid in the tank by perhaps as much as a 100 gallons or more. Such variation is undesirable economically and can cause regulatory problems because federal regulations require that a tank be capable of being reconciled to within one percent (1%) of the total volume of the UST (.+-.130 gallons); otherwise, it must be taken out of service.
There is, therefore, a need for an apparatus and method capable of sensing any deviation from level and further, for quantifying that deviation so that the strapping chart for an individual tank can be corrected to account for that tilt so as to provide more accurate inventory reconciliation and volume calculation from measurement of the depth of liquid in the tank. It is a primary object of the present invention, therefore, to provide such an apparatus and method. Other objects, and the advantages, of the invention will be apparent to one skilled in the art from the following description of a presently preferred embodiment thereof.